Rollup merge of #131418 - coolreader18:wasm-exc-use-stdarch, r=bjorn3

Use throw intrinsic from stdarch in wasm libunwind

Tracking issue: #118168

This is a very belated followup to #121438; now that rust-lang/stdarch#1542 is merged, we can use the intrinsic exported from `core::arch` instead of defining it inline. I also cleaned up the cfgs a bit and added a more detailed comment.